Session 2: Demonstrating that drag is influenced by the viscosity of the fluid

Objective: experiment the viscosity of a fluid

Explanation

the concept of viscosity seems well known by the students, but they will have a better understanding after few simple experiments

Material:

• few tanks
• different forms: flat plate, few balls, an aerodynamic form
• water
• oil
• honey
• shower gel
• few cylinders

Maximum duration:

20 minutes

Main questions to be asked:

• Give examples of liquids more viscous than water.
• How viscosity depends on temperature?
• Is viscosity influencing the drag? Why is that?

Introduction/Starters

Ask your students what is more viscose: water, oil or honey? Ask them to explain their choice.

Main activities

Ask the students to fill the three cylinders with water, oil and honey. After doing that ask them to open the orifice and let the fluid flow. Ask them which of the three fluids they think is less viscose and more viscose.

Liquids with higher viscosities will not make such a splash when poured at the same velocity.

Ask the students to fill with water and honey two tanks. Ask them to use a stick to blend the water and the honey. Then ask them to drop an abject in each of the tanks (two identical objects). Ask them to write their observations, which of the two fluids is more viscose and why.

Viscosity depends on temperature, when temperature increases the viscosity tends to decrease. The viscosity of air depends mostly on the temperature. To visualize this dependence, ask the students to heat the honey, they will observe the change in viscosity, the heated honey will be less viscous; also they can put the honey in the refrigerator for few minutes, and they will observe the change in viscosity, the honey in the refrigerator will be more viscous.

Ask the students to fill with water, oil and honey three tanks and to pick a form they want and to put it first
in the tank filled with water and try to move it and then do the same in the other two tanks. Put them work in groups of four. Ask them to write their observations.

Conclusion / Plenary

• Liquids with less viscosity will not oppose to movement.
• Liquids with higher viscosities will not make such a splash when drop an object in them.
• Drag is influenced by the viscosity of the fluid. If the fluid is more viscose, drag is increasing, because viscosity opposes movement.

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